From transactions of the ASME, Vol. 107, October 1985, p. 856-860 and Verfahrenstechnik 1-2 (1985), p. 18-21, processes are known in which pollutants like sulphur compounds, in particular hydrogen sulphide, as well as hydrogen chloride and hydrogen fluoride, are removed from the crude gaseous fuel after the gasification step in a wet scrubber while dust is removed in the electric precipitator. A disadvantage of the wet scrubbing process is the fact that the crude gaseous fuel is cooled down and waste waters are produced, which are problematic can contribute to environmental pollution.
As purification at higher temperatures improves the economy of the combined process, methods for hot gas purification have already been suggested. An insufficient cooling of the gaseous fuel may, however, cause difficulties in the gas turbine due to presence of vaporized alkalies, Powder Technology 40 (1984), p. 81-111. It is true that proposals for the separation of sodium and potassium at higher temperatures have already been worked out, but these processes complicate the entire process.
Moreover, purification at extremely high temperatures requires a separation of the hydrogen sulphide with sorbents, in particular lime. The regeneration problems in this context have not yet been solved, as can be appreciated from Report number ICTIS/TR03, IEA Coal Research, London 1979, chapter 2. A deposition of these products would create new environmental problems.